1. The Field of the Invention
The present invention is directed to retrograde cardioplegia catheters and the methods of their use and manufacture. More particularly, the catheters of the present invention are designed for rapid and accurate insertion into the coronary sinus and for retrograde administration of cardioplegia with maximum effectiveness and minimum tissue damage.
2. The Prior Art
Since the early days of cardiac surgery, it has been recognized that in order to provide the optimum surgical conditions when operating on the heart, it is necessary to interrupt the normal operation of the heart. For obvious reasons, an arrested, flaccid heart is preferred during a cardiac surgical procedure over a beating heart with blood s flowing through it. Thus, in order to be able to efficiently perform cardiac surgery, it is often necessary to use cardiopulmonary-bypass techniques and to isolate the heart from its life-giving blood supply.
It has been found that many deaths occurring after cardiac surgery are due to acute cardiac failure. At first, it was believed that the heart was simply beyond repair and that the operation had failed to correct the problem. Later, it was discovered that many of these postoperative deaths were due to new, and often extensive, perioperative (during or within 24 hours after the surgical procedure) myocardial necrosis (death of the heart tissue). Furthermore, many patients who survived were found to have suffered myocardial necrosis to a significant degree, thereby resulting in low cardiac blood output.
It is now known that myocardial necrosis occurs because the energy supply or reserve of the cardiac muscle cells is inadequate to supply the needs of the heart. The availability of oxygen dramatically affects the cell s ability to satisfy these energy requirements. For example, anaerobic metabolism of glucose produces two (2) moles of adenosine triphosphate ("ATP") per mole of glucose (as well as harmful acid metabolites), whereas aerobic metabolism of glucose produces thirty-six (36) moles of ATP per mole of glucose. Therefore, one of the primary goals of myocardial s preservation techniques during surgery is to reduce myocardial oxygen consumption.
Myocardial oxygen consumption is significantly reduced by stopping the electromechanical work of the heart. The oxygen demands of the beating empty heart at 37.degree. C. are four to five times those of the arrested heart (i.e., 4-5 ml/100-gm/min compared with 1 ml/100-gm/min). Buckberg, G. D., "Strategies and Logic of Cardioplegic Delivery to Prevent, Avoid, and Reverse Ischemic and Reperfusion Damage," 93 The Journal of Thoracic and Cardiovascular Surgery, 127, 136 (Jan. 1987) (hereinafter referred to as: Buckberg, "Strategies and Logic of Cardioplegic Delivery").
Oxygen consumption can be reduced further by cooling the heart. For example, the oxygen requirements of the arrested heart at 20.degree. C. are 0.3 ml/100-gm/min and are reduced to only 0.15 ml/100-gm/min at 10.degree. C. On the other hand, the oxygen requirements of the beating or fibrillating heart at comparable temperatures, are 2-3 ml/100-gm/min. Buckberg, "Strategies and Logic of Cardioplegic Delivery" at 129.
The normal heart receives its blood supply through the left and right coronary arteries which branch directly from the aorta. Generally, the veins draining the heart flow into the coronary sinus which empties directly into the right atrium. A few veins, known as thebesian veins, open directly into the atria or ventricles of the heart.
One of the early methods utilized to protect the myocardium during surgery was normothermic perfusion of the empty beating heart. This method was utilized in an effort to maintain the heart, as near as possible, in normal conditions during surgery. Although the procedure eliminated the problem of blood flow, dissection and suturing were still difficult to perform because of the firmness of the myocardium and the beating of the heart. Additionally, it was found that a significant amount of damage still occurred to the myocardium when this procedure was utilized.
A second method which was developed to protect the myocardium was intermittent cardiac ischemia with moderate cardiac hypothermia. This method requires that the entire body be perfused at a temperature of from 28.degree. C. to 32.degree. C., thus slowing all bodily functions, including those of the heart. The heart is fibrillated before aortic crossclamping to stop the beating. The surgeon can then operate for approximately fifteen to twenty-five (15-25) minutes, after which time the heart beat is necessarily resumed for three to five (3-5) minutes. This procedure proved to be an inefficient method for performing operations and had many attendant dangers, including the fibrillation of the heart.
A third method which has been utilized is profound hypothermic cardiac ischemia. This method requires that the temperature of the heart be lowered to about 22.degree. C. by the infusion of a cooled perfusate and/or by filling the pericardium with cold saline solution. One of the major disadvantages of this technique is that the heart continues to fibrillate, exhausting the heart's stored energy. As a result, the heart becomes acidotic, which over time causes irreversible muscle damage.
A fourth method which has been developed to preserve the myocardium during surgery is the infusion of a cold cardioplegic fluid to cool and stop the beating of the heart After the initial infusion, the heart is reperfused approximately every thirty (30) minutes to maintain the cool, dormant state of the heart.
The use of cardioplegia to protect the myocardium has proven the most advantageous method of those used to date. Cardioplegia, which literally means "heart stop," may be administered in an antegrade manner (through arteries in the normal direction of blood flow), in a retrograde manner (through veins opposite the normal blood flow direction), or in a combination of retrograde and antegrade administration. Cardioplegic solutions, typically containing potassium, magnesium procaine, or a hypocalcemic solution, stop the heart by depolarizing cell membranes.
Cardioplegia may be induced immediately after extracorporeal circulation has begun, provided that the pulmonary artery is collapsed to attest to the adequacy of venous return. In normal antegrade cardioplegia, a single venous return catheter is inserted in the right atrium to transfer the blood from the body to the heart-lung machine which a single needle is inserted into the aorta beneath the clamp through which the cardioplegic solution is administered. The cardioplegic solution flows through the coronary arteries in the normal blood flow direction.
If aortic insufficiency exists (imperfect closure of the aortic valve) or the patient is undergoing aortic valve replacement, then direct cannulation of the coronary arteries is necessary to perform antegrade cardioplegia. In this technique the aortic root is opened (using the procedure called "aortotomy") and perfusion catheters are inserted into both the left and right coronary ostia.
Care must be taken to avoid mechanical injury to the coronary ostia which could produce the serious complications of coronary ostial stenosis (i.e. constricting of the coronary ostia). Ostial stenosis requires reparative surgery and can be quite hazardous due to obstruction of the coronary arteries. Moreover, it is a nuisance to have perfusion catheters present within the limited operative field during aortic valve replacement. The inconvenience and time consumed by positioning perfusion catheters have led to dissatisfaction with direct coronary perfusion.
The foregoing risks and inconvenience of direct coronary cannulation may be avoided by using the retrograde cardioplegia technique. For this reason, some surgeons select retrograde cardioplegia as the preferred method of myocardial protection during aortic valve replacement.
Retrograde cardioplegia is conventionally administered by inserting a balloon catheter within the coronary sinus, inflating the balloon to stop the normal fluid flow into the right atrium, and perfusing the cardioplegic solution backwards through the coronary veins. In order to insert the catheter into the coronary sinus, the right heart must be isolated. To isolate the right heart, both the superior and inferior venae cavae must be tied and each must be cannulated. Once the right heart is isolated, the right atrium may be opened without allowing air to enter the circulatory system, thereby reducing the risk of systemic air embolization.
With the right atrium open, the catheter is visually inserted into the coronary sinus and hand-held while the cardioplegic solution is administered. The right atrium is then closed. This process must be repeated each time cardioplegic solution is administered during the operation. See Buckberg, "Strategies and Logic of Cardioplegic Delivery" at 132-33.
Retrograde cardioplegia is more complicated than antegrade cardioplegia because it requires right heart isolation, right atriotomy (i.e. opening the right atrium), and hand-holding the catheter during perfusion. Furthermore, retrograde cardioplegia may result in undesirable consequences.
For example, the atriotomy may lead to heart arrhythmia, and repeated cannulation may substantially injure the coronary sinus. In addition, high perfusion pressure or the failure to periodically allow normal venous drainage may damage the coronary veins and microcirculatory system causing edema. For these reasons, some surgeons completely avoid retrograde cardioplegia.
Nevertheless, there are some situations where retrograde cardioplegia is advisable over antegrade. For example, antegrade cardioplegia produces nonhomogeneous cooling and cardioplegic maldistribution in cases of myocardial ischemia and diffuse coronary disease. Antegrade cardioplegia does not adequately protect those areas of the heart downstream from coronary artery obstructions.
Several surgical graft techniques have been developed to circumvent coronary artery obstructions. In almost all of these techniques, cardioplegic solution is delivered down the grafts after they are completed. The graft is first attached to the coronary artery below the blockage, thereby leaving the other end of the graft open through which the cardioplegic solution can be administered. The open end of the graft is then attached to the aorta. Unfortunately, the area of the heart downstream of the obstruction does not receive any cardioplegic protection until after the graft is attached.
In the case of diffuse coronary artery disease, not all of the coronary blockages receive grafts. Therefore, the areas that are not grafted receive very minimal protection. In these situations, only retrograde cardioplegia can adequately protect those areas of the heart downstream from the coronary blockages.
Recently, some surgeons have begun using the internal mammary artery as the preferred graft for use on patients with coronary artery disease. It has been found that the internal mammary artery provides a superior long-term graft over the customary vein grafts (e.g., saphenous vein grafts). However, because the internal mammary artery remains proximally intact and insertion of a needle into the mammary artery would severely damage the artery, antegrade cardioplegia cannot be delivered through the internal mammary artery.
Many surgeons choose not to use internal mammary grafts in patients who have more severe forms of heart disease because antegrade cardioplegia is not available to protect the heart, notwithstanding the graft's superiority. Because antegrade cardioplegia does not adequately protect the heart downstream of the graft, that part of the heart muscle may be permanently damaged, resulting in a mortality or a very complicated, prolonged convalescence.
Although retrograde cardioplegia would provide adequate protection for those patients undergoing an internal mammary graft, surgeons often opt to use antegrade cardioplegia in combination with the inferior saphenous vein graft in order to avoid the cumbersome retrograde cardioplegia technique. The net result is that the sick patient receives a good short-term benefit by surviving the operation. But many years later, the patient has an inferior graft which may require additional surgery.
Furthermore, it has been found that by combining retrograde and antegrade cardioplegia many of the limitations inherent in the two protection strategies may be overcome so that a more uniform degree of myocardial hypothermia and complete regional and global left and right ventricular functional recovery is possible. Nevertheless, clinical adoption of retrograde cardioplegic techniques, alone or in combination with antegrade techniques, has been slow despite evidence of its usefulness. The principle reason for this delay in clinical acceptance seems to stem from the more cumbersome operative technique that is required to employ retrograde cardioplegia.
Most cardiac operations in adult patients are performed with single venous cannulation. Thus, the need for double cannulation of the venae cavae and isolation of these vessels, right atriotomy, and hand-holding of the catheter in the coronary sinus are all additional surgical procedures required in order to perform retrograde cardioplegia. These additional procedures, combined with possible isolation of the pulmonary artery, slower time to arrest, and possible large volumes of the cardioplegic solution needed to fill the right heart have limited the acceptance of current retrograde techniques.
In summary, retrograde cardioplegia often can provide superior myocardial protection over antegrade cardioplegia alone and the combination of retrograde cardioplegia and antegrade cardioplegia can provide superior myocardial protection than either technique alone. Yet there is substantial resistance by many surgeons to take advantage of the benefits of retrograde cardioplegia because it complicates an already complex surgical procedure.
From the foregoing, it will be appreciated that what is needed in the art are apparatus and methods for performing retrograde cardioplegia which are simple and effective so that the advantages of retrograde cardioplegia can be readily utilized by surgeons.
Additionally, it would be a significant advantage over the art to provide apparatus and methods for performing retrograde cardioplegia which do not require right atrial isolation, right atriotomy, and repeated cannulation of the catheter.
It would be another advancement in the art to provide a retrograde cardioplegia catheter which can be quickly and accurately inserted within the coronary sinus with relatively little trauma to the patient.
It would be yet another advancement in the art to provide apparatus and methods for performing retrograde cardioplegia which allow surgeons to safely use the internal mammary graft without making the surgical procedure cumbersome.
The foregoing, and other features and objects of the present invention, are realized in the retrograde cardioplegia catheter apparatus and method which are disclosed and claimed herein.